This application is to buy a new, high-performance fluorescence-activated cell sorter to serve a diverse group of clients at the California Institute of Technology Flow Cytometry/Cell Sorting Facility. The user group currently served by the Facility includes not only immunologists, but also developmental neurobiologists, genomics experts, structural biochemists, and chemical engineers using bacterial populations to model evolutionary logic. This range of research demands many distinct capabilities of a cell sorter, including the ability to sort rare populations quickly, robust multicolor analysis, the ability to sort bacteria and irregularly shaped cells, and the ability to excite and detect fluorescence resonance energy transfer between genetically engineered fusion proteins for tests of protein-protein interactions within live cells. These diverse applications call for a high-performance, modern cell sorter, and some of them cannot be served by any existing Facility instrumentation. The Facility presently owns two cell sorters and an analyzer, but one of the sorters is over 13 years old and incapable of providing important services. The difficulty of obtaining sorting time has become a major constraint on experimental progress. This application is to replace the old sorter with a new sorter that will improve service to existing clients and permit the Facility to carry out new flow cytometric applications that have not been available to the user group before. [unreadable] [unreadable] [unreadable]